1. Field of the Invention
The present invention relates to air bearing drive systems, and more particularly, it relates to a mechanism of the drive system for controlling a gap of an air film (hereinafter, a gap of an air film is simply referred to as an air film gap) formed by an air bearing.
2. Description of the Related Art
Roundness testers are conventionally used for obtaining a variety of data about geometrical round precision of a cylindrical form such as roundness, concentricity, and coaxiality. By placing a workpiece on a turntable, rotating the workpiece by the turntable, and detecting a profile of the workpiece by a detection head, the roundness testers collect data about the profile of the workpiece, and then measure and calculate the geometrical round precision of the workpiece.
As disclosed, for example, in Japanese Unexamined Patent Application Publication Nos. 2000-120686 and 2000-348429, air bearings producing dramatically less frictional resistance, heat generation, and rotating vibration than ball bearings and the like are widely used for a variety of accurate drive systems, for example, for achieving a rotary motion of the turntable and a linear feed motion of the detection head of the roundness testers.
In general, a rotary drive system 10 using an air bearing shown in FIG. 1 has a stator (also referred to as base) 12, a rotor 14, an upper plate (also referred to as moving portion) 16, and a lower plate 18. The rotor 14 is formed integrally with the upper plate 16 and lower plate 18 and supported by the stator 12.
By supplying air 26, the lower surface of the upper plate 16 and the upper surface of the stator 12 have an upper air film gap 20 formed therebetween, the lower surface of the stator 12 and the upper surface of the lower plate 18 have a lower air film gap 22 formed therebetween, and the outer peripheral surface of the rotor 14 and the inner peripheral surface of the stator 12 have a tubular air film gap 24 formed therebetween. Thus, these gaps 20, 22, and 24 allow the rotor 14 to rotate smoothly together with the upper and lower plates 16 and 18 relative to the stator 12.
The drive system using the foregoing conventional air bearing has an air film gap generally formed between the base and each of the moving parts facing the base. For example, in the rotary drive system 10 shown in FIG. 1, the air flows in these three gaps 20, 22, and 24. In order to achieve an accurate rotation of the rotor 14 together with the upper and lower plates 16 and 18 while maintaining all the air film gaps, between the base and the moving parts facing the base, at respectively fixed amounts of spacing, all the foregoing upper and lower gaps 20 and 22 and tubular gap 24 are required to satisfy a large area of extremely strict precision, as described below, for example.
Within currently attainable levels of geometrical precision, for example, the stator 12 is required to satisfy flatness of its upper and lower surfaces, parallelism between the opposing surfaces (hereinafter, also referred to as surface-to-surface parallelism), squareness of the axis of its inner peripheral surface relative to the above surfaces, cylindricity of the inner peripheral surface, and so forth. Also, the rotor 14 is required to satisfy flatness of its upper and lower surfaces, parallelism between these surfaces, squareness of the axis of its outer peripheral surface relative to the above surfaces, cylindricity of the outer peripheral surface, and so forth. In addition, the upper plate 16 is required to satisfy flatness of its upper and lower surfaces and the like, and the lower plate 18 is required to satisfy flatness of its upper surface and the like.
However, since satisfying the foregoing variety of extremely strict geometrical precision requires a large amount of work and time for processing and finishing these components, and accordingly incorporates an increased cost of the drive system, a new drive system which can achieve a reduced cost has been strongly desired without deteriorating accurate and stable driving features required.
Furthermore, when the center of rotation and the center of gravity of a rotating workpiece placed on the upper plate 16 for measuring its roundness do not coincide with each other, an eccentric load is exerted on the rotating upper plate 16, causing the air film between the lower surface of the upper plate 16 and the upper surface of the stator 12 to vary in accordance with the rotation of the rotating workpiece, giving rise to a problem in that it is difficult to accurately measure geometrical round precision of the workpiece because the axis of the rotation center of the workpiece is tilted.
In view of the above-described problems of the related art, it is an object of the present invention to provide an air bearing drive system that offers a reduced cost and achieves accurate and stable driving performances.
The present invention is made to achieve the above object. An air bearing drive system according to the present invention comprises (a) a moving portion performing a rotational motion or a linear motion, (b) a base for supporting the moving portion, (c) an air bearing having an air film gap, for supporting the moving portion on the base, between the base and the moving portion, (d) at least one air nozzle, (e) air supplying means, (f) at least one suction inlet, (g) attracting means, and (h) two adjusting means.
The air nozzle is formed on the base and faces the moving portion, for forming the air film gap by blowing air toward the moving portion and exerting a levitation force on the moving portion. The air supplying means supplies air to the air nozzle.
The suction inlet is formed on the base and faces the moving portion, for exerting an attraction force on the moving portion so as to attract the moving portion toward the base. The attracting means attracts the moving portion toward the base via the suction inlet.
The two adjusting means adjust the levitation force produced by the air blowing from air nozzle and the attraction force from the suction inlet.
According to the present invention, by adjusting the thickness of the air film between the moving portion and the base by using the air nozzle and the suction inlet, the thickness of the air film can be fine-tuned or adjusted without making increasing the size of the air bearing.
Further, in the air bearing drive system according to the present invention, it is preferable that the moving portion comprises a cylindrical rotor and a moving flat surface orthogonal to the axial center of the rotor, the base comprises at least one thrust flat surface facing the moving flat surface, and the thrust flat surface comprises the air nozzle and the suction inlet formed thereon so that the moving flat surface is levitated, at a prescribed height via the air film, relative to the thrust flat surface.
According to the present invention, the moving portion can be easily positioned in a non-contact manner since the moving portion has a levitation height controlled in the thrust direction thereof.
Further, in the air bearing drive system according to the present invention, the thrust flat surface is preferably a single surface disposed on one side of the stator.
Since the air bearing drive system according to the present invention has a smaller number of elements of components requiring precise processing, unskilled workers can process the components. Accordingly, a reduced processing cost of the components is achieved without a risk of deteriorating rotation accuracy of the drive system.
Further, in the air bearing drive system according to the present invention, the base may have a cylindrical opening therein for supporting the rotor in the radial direction of the rotor. Also, the base may comprise a plurality of the air nozzles disposed on the peripheral surface of the cylindrical opening so that the rotor is positioned in the center of the cylindrical opening.
According to the present invention, since the rotor is positioned in the center of the cylindrical opening in a non-contact manner, the air bearing drive system has an integrally formed radial and thrust bearing, leading to a reduced size of the drive system.
Further, in the air bearing drive system according to the present invention, either the air nozzle or the suction inlet may have a groove-shape.
According to the present invention, since the thickness of the air film between the base and the moving portion can be controlled easily and a variation in the thickness can be reduced accordingly, the drive system has improved driving accuracy.
Further, in the air bearing drive system according to the present invention, the moving portion may further comprise an anti-slip member for preventing the moving portion from slipping out from the base.
According to the present invention, a possibility of an accident during conveyance and the like can be eliminated.
Further, the drive system according to the present invention is preferably used for driving a turntable of a roundness tester.
Accordingly, the present invention provides a roundness tester comprising a precise rotary driving mechanism at a low cost.
Further, the air bearing drive system according to the present invention may further comprise controlling means for controlling the adjusting means to adjust a balance of the levitation force and the attraction force exerted on the moving portion so that the air film gap between the base and the moving portion maintains a required uniform thickness when the moving portion has the levitation force and the attraction force exerted thereon, wherein (i) at least one group of the plurality of the air nozzles and the plurality of the suction inlets is disposed on the base, (ii) the corresponding adjusting means independently adjusts at least one of a part of the levitation force by using each of the air nozzles and a part of the attraction force by using each of the suction inlets, and (iii) the control means controls the corresponding adjusting means to perform the above-stated independent adjustment so that the air film gap between the base and the moving portion maintains the required uniform thickness when the moving portion has the levitation force and the attraction force exerted thereon.
The required uniform thickness of the air film gap as described above is achieved by adjusting surface-to-surface parallelism and the like of the air film gap, for example, by adjusting a height and an angle of the moving portion relative to the base.
According to the present invention, even when an eccentric load is exerted on the rotating moving portion when the center of rotation and the center of gravity of a rotating workpiece placed on the moving portion for measuring its roundness do not coincide with each other, the air film between the lower surface of the moving portion and the upper surface of the base is controlled so as to have a constant thickness in accordance with a rotation of the rotating workpiece, thereby achieving an accurate rotation of the workpiece without tilting the axis of the rotation center of the workpiece.
Further, in the air bearing drive system according to the present invention, the control means may comprise at least one gap sensor, concentrically disposed with the peripheral circle of the rotor, for measuring a levitation height of the moving flat surface relative to the thrust flat surface.
According to the present invention, since the levitation height of the moving flat surface relative to the thrust flat surface can be measured accurately, a slanted angle of the moving flat surface can be calculated more accurately, allowing the control means to control the levitation height more accurately, thereby achieving a more accurate rotation of the workpiece.
Further, in the air bearing drive system according to the present invention, the control means may comprise at least one pressure sensor for measuring a negative pressure.
According to the present invention, since the pressure sensor for measuring a negative pressure is disposed in the air suction line, the levitation height of the moving flat surface relative to the thrust flat surface can be maintained constant regardless of the weight of the workpiece by adjusting the negative pressure so as to maintain the attraction force constant.
As described above, the drive system according to the present invention comprises (a) the base, (b) the moving portion, (c) at least one air nozzle formed on the upper surface of the base for blowing air toward the moving portion so as to exert a levitation force on the moving portion, (d) at least one suction inlet also formed on the upper surface of the base for attracting the moving portion toward the base so as to exert an attraction force on the moving portion, and (e) two adjusting means, one for adjusting the levitation force from the air nozzle and the other for adjusting the attraction force from the suction inlet. With this configuration, the adjusting means adjust a balance of the levitation force and the attraction force so as to form an air film gap having a required uniform thickness, thereby achieving a reduced cost as well as an accurate and stable drive of the drive system.
In the drive system according to the present invention, at least one group of a plurality of the air nozzles and a plurality of the suction inlets is disposed on the base. The corresponding adjusting means independently adjust at least one of a part of the levitation force by using each of the plurality of air nozzles and a part of the attraction force by using each of the plurality of suction inlets. Also, the drive system comprises control means for controlling the corresponding adjusting means to perform the above independent adjustment, thereby achieving further accurate and stable driving performances of the drive system.